This is not a site where you can sell photos, but it is a site that gives away free stock photos that turn out not to be accepted anywhere else.
This is where I learned photography initially - they have an excellent on-line photography course. There's no longer an on-line instructor, but the course is still there and anyone posting the homework assignments still get help for those who've taken the course, and from those professionals who frequent the site.
Those of you who have
good photos not picked up by other sites might want to share some of those...quite a lot of mine have been picked up by magazines, government agencies, educational publishers, etc. Most of my gallery there are pre-professional images, though there are a few I've posted since going professional - mostly reference images I thought would be useful in an editorial or educational way.
http://www.morgueFile.com
PS: you can also download free photos for your own uses for those of you who are also designers.